Dow STYROFOAM Rigid Foam Board

Rigid foam board is the go-to for adding continuous insulation to walls, foundations, and below-grade work where batt insulation alone won’t cut it in a Montana winter. The extruded polystyrene (XPS) version — Dow STYROFOAM — is one of the common choices, and it shows up in a tight, predictable lineup of sizes.

Here’s what XPS board looks like off the rack:

Product	Size	Stated R-value
STYROFOAM scored board, 1-1/2”	4’×8’	R-7.5
STYROFOAM board, 1”	4’×8’	R-5
STYROFOAM scored board, 2”	4’×8’	(R-value not listed)

That confirms the basics. XPS comes in 4’×8’ sheets at 1”, 1-1/2”, and 2” thicknesses, scored board is an option, and the rating climbs with thickness — roughly R-5 per inch.

A full rigid insulation buyer’s guide gets into a lot more: EPS and polyiso comparisons, edge profiles like tongue-and-groove and fanfold, ASTM classifications, below-grade drainage board, fastening schedules, and code thermal-barrier requirements. Those specifics aren’t nailed down here, and guessing at R-value selector numbers, fastener sizes, or code thresholds does a builder no favors. Better to confirm those details before specifying a board than to run with numbers that won’t hold up on the jobsite.

What’s solid is the size and R-value picture for STYROFOAM XPS — three thicknesses, the standard 4’×8’ footprint, and the option of scored board.

Frequently Asked Questions

Q: What thicknesses does Dow STYROFOAM rigid board come in?

Dow STYROFOAM extruded polystyrene (XPS) board comes in 4’×8’ sheets at 1”, 1-1/2”, and 2” thicknesses. Both flat and scored board options are made in this lineup.

Q: What’s the R-value of Dow STYROFOAM board?

The 1” Dow STYROFOAM board is rated R-5, and the 1-1/2” scored board is rated R-7.5. XPS holds roughly R-5 per inch, which is why thickness drives the rating.

Q: What’s the difference between STYROFOAM and polyiso?

The two are different rigid insulation chemistries — STYROFOAM is extruded polystyrene (XPS), while polyisocyanurate (polyiso, or PIR) is a separate foam type. Both exist as rigid insulation, but specs and a side-by-side R-value comparison vary by product, so confirm the numbers before specifying one over the other.

Q: What is scored STYROFOAM board used for?

Scored Dow STYROFOAM board is offered in the 1-1/2” (R-7.5) and 2” sizes in the 4’×8’ format. Verify the intended application before ordering to match the right board to the job.

Q: Does rigid foam board come in 4x8 sheets?

Yes — Dow STYROFOAM XPS board is made in the standard 4’×8’ sheet at 1”, 1-1/2”, and 2” thicknesses. That’s the same footprint as a sheet of plywood or OSB, which keeps layout straightforward on a wall or foundation.

Q: Is rigid foam board structural?

No. Rigid foam board is a non-structural insulation product, so it doesn’t replace code-required corner bracing or shear walls. Plan your structural sheathing and bracing separately from your foam layer.